Monitoring of Haemophilus influenzae isolated from carriage, lower respiratory tract infections and blood over a six-month period in Belgium.

Introduction: H. influenzae carriage may evolve into respiratory or systemic infections. However, no surveillancesystem is in place in Belgium to monitor carriage strains.

Material and methods: This study provides a detailed description of H. influenzae strains isolated from both carriage and lower respiratory infections, collected during a six-month national surveillance. Subsequently, a comparison is conducted with invasive isolates collected during the same period at the National Reference Centre (NRC).

Results and discussion: From November 2021 to April 2022, 39 clinical laboratories collected 142 and 210 strains of H. influenzae from carriage and infection, respectively, and 56 strains of blood were submitted to the NRC. In each group, the biotype II comprised more than 40%, followed by biotypes III and I. The majority of strains were non-typeable H. influenzae, with a notable increase in the number of encapsulated strains in the invasive group (14.3% vs. 1-2%). A beta-lactamase was identified in 18.5% and 12.5% of surveillance and invasive strains, respectively. Resistance to the amoxicillin-clavulanic acid combination accounted for 7% in the surveillance strains and 10.7% in invasive strains. The overall resistance to third-generation cephalosporins at 1.2% is consistent with rates observed in other European countries. Of particular significance is the identification of mutations in the ftsI gene in both carriage and infected strains, which are associated with high-level beta-lactam resistance.

Conclusion: NRC must engage in regular and systematic monitoring of beta-lactam susceptibility of H. influenzae to guarantee safe empiric therapy in severe cases and identify potential transitions from low-level to high-level resistance in the future.